It is basically known for hollow bodies such as for example fuel tanks, drums or heating oil tanks to be produced from plastic material by extrusion blow molding.
In that case generally granulated thermoplastic material is plasticized in one or more extruders and fed to an extrusion head. The plasticized material is expelled from the extrusion head in the form of a tubular preform which is either removed at the extrusion head and introduced by means of manipulators into an opened mold tool or is extruded into the mold tool in a hanging condition directly at the extrusion head. The mold tool generally comprises two mold halves forming a mold cavity defining the external contour of the finished blow-molded article. The preform is expanded within the mold cavity by means of gas pressure and/or by means of a reduced pressure. The finished article is of an external contour corresponding to the contour which is predetermined by the mold cavity of the tool.
It is equally known for a tubular preform, after extrusion, to be cut up to the intended final length into one or more material webs and for those material webs to be transported by means of a gripper into position between the opened halves of a tool.
Finally it is also known for preforms in web form or band form to be expelled from extruders with wide-slot nozzles and for the preforms to be shaped in some manner in the first or second heat to afford the definitive configuration of the finished product.
When dealing with articles of a comparatively complicated external contour a tubular preform is generally extruded. In that situation the material which is extruded at the extrusion head in a hanging condition experiences a certain amount of stretch by virtue of its length-related weight. If no measures are taken to compensate for that stretch, that results in a differing wall thickness distribution on the finished product, in the extrusion direction. When dealing with molten materials of comparatively low viscosity that can be counteracted by means of discontinuous extrusion.
It is easy to envisage that articles of a comparatively complex external configuration experience different degrees of stretching upon expansion at various locations within the blow molding mold so that this factor alone results in differing wall thickness distribution. In the production of certain products different wall thicknesses on the finished product are desirable, whereas other products require different wall thickness profiles for reasons of stability in the finished product. Finally it is known for the wall thickness of the extruded preform to be varied in such a way that differing degrees of stretch are compensated upon expansion of the preform. Critical articles in that respect are transparent detergent containers, canisters with handles shaped therein and so forth.
For that purpose it is known for a wall thickness profile to be imparted to tubular preforms, more specifically both axially and also radially. That is implemented for example by adjustment of the annular gap at the extrusion head. In the case of very large products wall thickness control is also used to compensate for the lengthwise stretch of the tube in the extrusion procedure.
It is also desirable in the case of relatively large products such as for example in the case of fuel tanks of plastic material to impart to the preform a wall thickness which varies over the length and/or periphery thereof, so that differing degrees of stretch in the tool can be compensated or it is possible to achieve a different wall thickness distribution on the finished product. It is conceivable for example that, in the case of a fuel tank, a different wall thickness distribution is to be provided for reasons of stability in order to be able to better accommodate any impact-induced deformation.
Therefore the object of the present invention is to provide a process for the production of an extrusion blow molded container of thermoplastic material, which entails very substantial freedom in terms of variation in the wall thickness of the container in any direction.